This invention relates to improvements in motor drive circuitry, especially but not exclusively for use in electric power assisted steering assemblies.
Electric power assisted steering systems are known of the kind in which an electric motor applies assistance torque to a part of the steering in order to assist the driver of the vehicle in turning the wheel. The motor may typically act upon the steering column or may act upon the steering rack, through a reduction gearbox. A measurement of the torque applied to the steering apparatus by the driver when turning the wheel is passed to a processor which produces a torque demand signal which is in turn used to control the motor to produce the required assistance torque. Applying an assistance torque of the same sense as the driver applied torque reduces the amount of effort needed to turn the wheel.
The motor, which may be a star or wye connected motor, such as a multiphase permanent magnet motor, is controlled by a motor control circuit and a motor drive circuit. The motor drive circuit comprises switches which can be opened and closed to connect the phases of the motor to a DC source, such as a battery or an earth, in response to a control pattern provided by the control circuit. Specifically, each phase is connected to a positive supply rail through a top transistor which when turned on connects the motor phase to a battery positive terminal connected to the positive supply rail. Similarly, each phase is connected through a bottom transistor to a negative supply rail through a bottom transistor. When switched on, the bottom transistor connects the phase to the negative rail which is in turn connected to a battery negative or earth. The two transistors—top and bottom—form one arm of a multiple arm bridge circuit that is the heart of the drive circuit. By opening and closing the switches it is possible to selectively and independently route current through each phase of the motor.
The control circuit comprises a digital or analogue circuit or some combination of both. The function of the control circuit is to supply control signals to the bridge transistors to open and close them in a pattern which in turn causes the current to flow through the phases as required for a given motor torque and speed. Generally the pattern will be set by the control circuit according to the motor position and the torque measured in the steering system by a torque sensor. Typically the pattern for each arm of the bridge comprises a pulse width modulated waveform.
An example of a typical prior art motor and drive circuit is shown in FIG. 1 of the drawings. A battery (not shown) supplies power to a 3 phase bridge with top switches 2, 3, 4 and bottom switches 5, 6, 7 which feed a 3 phase permanent magnet motor 8. The switches shown are MOSFETS but could be any other type of semiconductor switch such as Bipolar transistors. Where reference is made in this document to MOSFET devices the reader should understand that this is intended generally to cover any solid state relay or switch.
A problem with such an arrangement is that a fault mode can arise in which a top switch in an arm of the bridge and a bottom switch in another arm of the bridge may both be stuck in the closed position, resulting in a permanent path for DC current from the battery through the positive rail, through at least two phases of the motor and back to the negative rail. This can occur for many reasons, such as a fault in the control circuit resulting in a control pattern being applied to the drive circuit which is incorrectly instructing transistors to stay closed, or a faulty switch. When such a fault condition occurs the motor resists turning, making it difficult for the driver to turn the wheel.
To prevent the current being drawn from the supply along the path described in the previous paragraph, the remaining bridge switches could be placed in a fault mode where they are all turned OFF (i.e. open circuit). However, it is still possible for current to flow through the motor along a path as shown in FIG. 2 of the drawings. Due to the inductance of the Motor, any current flowing in the fault mode will continue to flow through the faulty bridge switch and the body diodes of two other top or bottom MOSFETs—dependant on the direction of current flow.
With no other source present, this fault current will decay to zero as energy is dissipated in the resistance of the motor windings and over the forward voltage drop of the conducting MOSFET body diodes. This is shown in FIG. 3.
However, this situation does not adequately isolate the Motor; continued (unassisted) steering input from the driver will rotate the motor, generating a back-emf voltage between windings. As soon as this back-emf exceeds the forward voltage of the MOSFET body diode (top or bottom MOSFET alongside the faulty MOSFET) current will again flow giving a half-wave rectified periodic current waveform, resisting the actions of the driver (Motor Damping). This is shown in FIG. 4 of the drawings.
This is an unacceptable situation which must be rectified within a short duration set by the applicable Safety Requirement.
To ensure that current cannot flow due to back EMF Vbemf1,2 as the motor is physically rotated, for example by a driver, it is known to place in each motor phase an additional isolation switch referred to in this text as a solid state phase isolation relay (SSPIR). This term encompasses a range of solid state switches including MOSFETS and bipolar transistors. When a fault has occurred, the drive circuit is placed in a fault event mode in which these switches are held open (non-conducting) to ensure no current can flow in the phase. A simple circuit with an isolation switch, herein referred to as a solid state phase isolation relay (SSPIR), in each phase, is shown in FIG. 5.
Although the use of SSPIRs would appear to be a perfect and total solution to the problem, the applicant has previously appreciated that an issue with an SSPIR arises when a SSPIR opens either intentionally or unintentionally whilst a high current is flowing through it. Under this circumstance, the voltage across the SSPIR will rise rapidly due to the increasing drain-source resistance in the moments before opening until the breakdown voltage of the switch is reached (avalanche condition), unless limited by external means. This combination of high voltage in the presence of high current flow results in a short high-power pulse. The energy contained within this pulse may result in the short-circuit failure of the SSPIR, defeating its purpose. This is shown in FIG. 6.
It is known to wait for the current in the motor to decay before opening the SSPIRs. However, this is not a total solution to the problem of possible short-circuit failure of the SSPIRs. Consider a representative automotive application based on a 12 volt supply battery where the bridge MOSFETS are opened as shown in FIG. 6 following detection of a faulty bridge MOSFET. Continued (unassisted) steering input from the driver will rotate the motor, generating a periodic back-emf voltage between windings. As soon as this back-emf exceeds the forward voltage of the MOSFET body diode (top or bottom MOSFET alongside the faulty MOSFET) current will again flow (half-wave rectified), resisting the actions of the driver (Motor Damping). This current may be high enough that any subsequent attempt to open the SSPIRs will cause them to enter an avalanche mode where the power dissipated exceeds the rating of the SSPIR, leading to catastrophic failure.
Attempts have been made in the past to overcome this limitation by providing a snubber circuit that absorbs the energy during an opening of an SSPIR that would otherwise lead to damage due to excessive power dissipation in the SSPIR. However, the provision of the snubber circuit in itself can cause problems where the snubber circuit cannot deal with the sudden changes in current, and in any event the introduction of additional components in the snubber increase costs and lead to more potential points of failure.
An object of the present invention is to ameliorate the problems associated with the use of SSPIRs without resorting to the use of additional snubber circuitry.